U.S. Pat. No. 6,246,144 to Hockaday, et al., incorporated herein in its entirety by this reference, illustrates an exemplary holder assembly for two carbon brushes. A pair of coil springs is included within the assembly, with each spring biasing an associated brush toward the surface of a commutator or other rotating machine element. Because the coil springs likely are designed only for light-duty use, the force provided by such springs presumably may easily be overcome by compressing the springs manually.
By contrast, brush holders intended for use with locomotives, for example, must withstand much heavier duty. Consequently, their designs differ significantly from that of the Hockaday patent. Moreover, the severity of the environment in which train-mounted brush holders are placed has precluded use of conventional constant-force holders for these purposes. Instead, variable-force holders traditionally are used, notwithstanding that such holders could result in non-uniform performance and life, elevated brush wear, sparking, and increased potential for flash-over.
Drive motors for locomotives, furthermore, typically are integral with the drive trains and mounted on the undercarriages of the locomotives. Significant spatial constraints thus exist for the associated brush holders in connection both with their placement for use and their removal for brush replacement. To accommodate these constraints, “finger”-style brush holders have been created. Finger-style holders have low profiles and are easily disengaged from the corresponding brushes so as to facilitate replacement thereof when worn to unacceptable levels. They are, however, non-constant-force devices, as the spatial constraints prevent easy retraction of the their springs when brushes need to be removed and replacement brushes and spring brackets need to be returned to the holders. Indeed, in the past, extraordinary manual force has been required to counteract the spring forces and push spring brackets into brush holders.